The present invention relates to closures in general, and more particularly to improvements in closures in the form of caps which are made of a synthetic plastic material and can be used as a means for sealing the necks of bottles or like containers. Still more particularly, the invention relates to improvements in internally threaded plastic caps which can mate with external threads on the necks of bottles and like containers.
It is well known to provide the end wall or bottom wall of a cup-shaped plastic cap with an internal seal which abuts against the end face of the neck when the cap is used to seal the open end of a bottle or the like. As a rule, the inner side of the bottom wall of the cap carries a disc of soft elastomeric material which is bonded to or is formed as an integral part of the cap. A drawback of such caps is that their making necessitates at least two discrete operations which contributes significantly to the manufacturing cost. Another drawback of conventional caps with disc-shaped sealing elements at the inner sides of their end walls is that such caps cannot undergo a sterilizing treatment and also that their material cannot be reused by recycling.
It was further proposed to make the cap from a single piece of synthetic plastic material and to reinforce the region between the end wall and the internally threaded tubular skirt of the cap by one or more circumferentially complete ribs having different dimensions. The ribs are adjacent to that end of the internal thread in the skirt which is nearest to the end wall and they are supposed to perform the function of sealing lips by coming into more or less pronounced engagement with the exterior of the neck forming part of a bottle which carries the cap. Reference may be had to British Pat. No. 960,443 which further shows a pronounced horizontal rib between the inner end of the internal thread and the inner side of the end wall of the cap. The manner of forming such pronounced ribs and especially of removing caps which embody such pronounced ribs from an injecting or other mass-producing machine is not disclosed in the patent.
Caps of the type disclosed in British Pat. No. 960,443 are intended for confinement of non-expandible goods and/or for confinement of commodities (such as beer) which are likely to be maintained at a pressure only slightly above the pressure of air in the surrounding atmosphere. However, the patented caps cannot be used on containers which confine goods requiring sterilization, pasteurization or a similar treatment.
British Pat. No. 925,647 discloses a method of making a modified closure or cap. The patented cap exhibits the drawback that its removal or expulsion from the making machine is a difficult and complex operation. The machine must be equipped with a specially designed injecting tool and the circumstances under which the cap is made must be monitored and regulated with a very high degree of precision. This, too, contributes to high initial cost of the patented cap.
British Pat. No. 925,647 already shows a cap which is formed with a curved bottom wall or end wall. The exterior of the transition zone between the end wall and the internally threaded skirt of the patented cap makes an obtuse angle and has a pronounced edge. Such edge is disposed radially outwardly of a thin flexible lip at the inner side of the cap. When the pressure in the interior of the container which carries the just discussed cap increases (e.g., in the course of a pasteurizing or sterilizing operation), stresses which develop in the region between the end wall and the skirt reach a value which is a multiple of the stresses in the end wall or in the skirt proper. Moreover, tension in the marginal portion of the end wall deviates very substantially from tension in the skirt. This results in radial expansion which is increased as a result of heating (e.g., for a period of up to 60 minutes within an autoclave which is used for sterilization or pasteurization of the contents of a bottle or another container carrying the patented cap) so that the cap is incapable of preventing communication between the interior and the exterior of the container. Moreover, the cap is likely to crack in the region of the open end of the skirt as a result of pronounced internal stresses and/or as a result of a pronounced pressure differential between the interior and the exterior of the container.